1. Field of the Invention
The present invention relates to an image forming device, and more particularly relates to an image forming device that includes a developing device of a so-called trickle system which supplies a new developer while discharging a degraded developer little by little.
2. Description of the Related Art
In a developing unit using a two-component developer containing a toner and a carrier, since the toner is consumed by image formation but the carrier remains within the developing unit, the carrier is degraded with time. Hence, in recent years, much attention has been focused on the so-called trickle system which supplies a new developer to the developing unit while discarding, little by little, the developer containing the carrier from the developing unit, and various proposals have been made.
For example, in Japanese Unexamined Patent Publication No. 2007-52213, in order to reduce variations in the properties of a developer caused by the difference between a print mode and a print coverage, a technology is proposed in which a control voltage of a toner concentration detection unit is corrected based on a developing member operating time and the amount of carrier supplied.
Incidentally, the weight of the developer supply bottle of the trickle system is increased by a weight corresponding to the carrier contained therewithin, as compared with a supply bottle that supplies only a toner. Normally, the weight proportion of the carrier in the developer supply bottle of the trickle system is 10% to 20%, and since the amount of carbon dioxide discharged is increased accordingly, a load to the environment is increased. Hence, studies have been conducted where in order to reduce the load to the environment, as the carrier within the developer supply bottle, a carrier whose bulk density is less than that of the carrier within the developing unit is used.
However, a toner concentration within the developing unit is detected with, for example, a detection unit such as a permeability sensor, and based on its detection value, the supply of the developer from a supply container is controlled such that the toner concentration is kept within a predetermined range, but when the physical property of the carrier supplied to the developing unit is different from that of the carrier within the developing unit, a correlation between the actual toner concentration within the developing unit and the detection value of the permeability sensor is degraded, with the result that even if the detection value of the permeability sensor remains the same, the actual toner concentration within the developing unit may be varied. When the toner concentration within the developing unit is varied, a toner charge amount is varied, and thus a failure such as a reduction in image concentration, fogging, uneven density or scattered toner may be produced.